Although compounds corresponding to formula (I) above, which are primarily used as flame-retarding additives for plastics, particularly polyurethanes, have been mentioned in connection with higher homologues of this class of compounds, the preparation thereof by known methods does not give satisfactory results (cf., for example, U.S. Pat. Nos. 3,076,010 and 3,297,796). For example, only the diethyl esters of bis-(hydroxyethyl)-aminomethane phosphonic acid have hitherto been available, being produced by reacting diethanolamine with aqueous formaldehyde and diethyl phosphate (cf., for example, German Auslegeschrift No. 1,143,022). So far it has not been possible to satisfactorily produce the corresponding dimethyl esters which, of course, would be of considerable interest.
Where compounds corresponding to the above formula are produced by conventional processes, secondary reactions occur to a considerable extent. Reference has been made in the literature, inter alia, to the alkylating effect of, in particular, lower alkyl phosphites, but also of dimethyl phosphonates on amines (Houben-Weyl, Methoden der organischen Chemie, Vol. 12/2, page 10 and Vol. 12/1, page 411).
The same occurs in the reaction of alkanolamines and formalin with dimethyl phosphite. In addition, the water present in the reaction of dialkanolamine and formalin hydrolyzes the phosphite component which, as a result, may no longer participate in the required reaction. Another secondary reaction is the transesterification of the methoxy group bound to the phosphorus with the terminal hydroxyl groups in the alkanolamine. The transesterification reaction is known to be promoted to a greater extent in the case of phosphites containing methyl groups than in the case of higher homologues.